As is known, in IGBT transistors the maximum current in the conduction state is limited by the presence of an intrinsic parasitic thyristor. For greater clarity, reference may be made to FIG. 1, which shows the structure of a conventional IGBT transistor, designated by the reference number 1. The IGBT transistor 1 comprises a drift region 2 of an N−-type, a body region 3, of a P−-type. In turn the body region 3 houses two symmetrical source regions 5, of an N+-type. The body region 3 and the source regions 5 are aligned to a surface 2a of the drift region 2 and are contacted by an emitter line 7. Gate regions 8 are arranged on the surface 2a, at sides of and partly overlying the body region 3, and are separated from the underlying structures by gate-oxide regions 9. More precisely, the gate regions 8 are each aligned to a respective one of the source regions 5, and portions of the body region 3 immediately underlying the gate regions 8 define channel regions 10. The gate regions 8 are directly connected to one another and are provided with a gate terminal (not shown). The IGBT transistor 1 further comprises a collector region 12, of a P+-type, separated from the drift region 2 by a transition region 13, of an N+-type. Finally, a collector contact 15 coats the collector region 13 throughout its extension.
From an electrical standpoint, the body region 3, the drift region 2, and the collector region 13 define a PNP transistor 17, controlled by an NMOS transistor 18 formed by the source regions 5, the body region 3 (in particular, the channel regions 10) and the drift region 2. In addition, a further parasitic NPN transistor 19 is present, formed by the source regions 5, the body region 3 (outside the channel regions 10) and, once again, the drift region 2. In practice, the PNP transistor 17, the MOS transistor 18 and the NPN transistor 19 form a thyristor 20. In use, the current of the IGBT transistor 1 is substantially determined by a conduction current ID (of holes, in this case) of the PNP transistor 17 and by a control current IC (of electrons), which flows through the NMOS transistor 18. The NPN transistor 19 should be normally off. The body region 3, however, has a low doping level and hence a relatively high internal resistance RI. Consequently, the conduction current ID may cause a voltage drop between the source regions 5 and the body region 3 (which form the emitter and the base, respectively, of the NPN transistor 19) such as to turn the NPN transistor 19 on. In this case, the thyristor 20 is activated, and the current flows directly towards the emitter line 7, independently of the MOS transistor 18, and hence can no longer be controlled by means of the gate terminal 11. In addition, activation of the transistor 20 limits the maximum current of the IGBT transistor 1, especially at high temperatures.
In order to overcome this problem, it has been proposed to reduce the resistance of the body region 3 by means of deep diffusion of a dopant species. In particular, the treatment concerns that portion of the body region 3 where mainly the conduction current ID is present. The dopant species is introduced through the openings between the gate regions 8 and diffuses towards the drift region 2. However, the diffusion is substantially isotropic and hence can easily alter doping of the channel region 10, in effect increasing the threshold voltage of the MOS transistor 18. The phenomenon hence limits the depth that the diffusion can reach and, consequently, also the benefit that can be achieved.
According to an alternative solution, a shallow implantation self-aligned to the gate regions 20 is performed and is followed by a diffusion for a short time. In this way, doping of the channel regions 10 and the value of the threshold voltage of the MOS transistor 18 are preserved, but it is possible to obtain only a modest reduction in the resistance of the body region 3.